A major focus in our studies of T cell differentiation is to identify the developmental signals that control cell fate decisions. In this regard, our most significant findings in this reporting period relates to how developing T cells decide to become alpha-beta or gamma-delta T cells and for those T cells that choose the alpha-beta T cell lineage, the mechanism underlying CD4 or CD8 lineage commitment. Others have proposed that it is the successful rearrangement of the alpha-beta or gamma-delta TCR genes that determine the alpha-beta or gamma-delta lineage choice. Our data suggest, however, that early rearrangement and expression of either TCR (alpha-beta or gamma-delta ) will allow gamma-delta lineage commitment. This result indicates that the lineage choice is not dictated by the class of TCR expressed. Moreover, some cells with productive rearrangement and expression of TCRgamma-delta do choose the alpha-beta pathway and upon successful rearrangement and expression of TCRbeta have a mechanism to silence gamma-delta TCR expression. Our more recent studies indicate that additional signals through the transmembrane receptor Notch, also influence this lineage choice. If and how TCR and Notch signals might be processed/integrated is currently under investigation. Taken together, our results demonstrate that while TCR gene rearrangement and expression is a necessary requirement for development it is not sufficient for alpha-beta/gamma-delta lineage committment. Also implied from these studies is that while the TCRalpha gene rearrangement process is terminated as a result of MHC engagement (which is also a requirement for maturation/positive selection of alpha-beta T cells), gamma-delta T cells have no requirement for MHC signals and MHC has no effect on gamma/delta rearrangement in gamma-delta committed cells. Our experiments indicate that it is the simple expression of a good TCR that terminates the TCR gene rearrangement process in this lineage. Defining factors that influence the CD4/CD8 lineage decision could be an important step toward resolving the underlying mechanism. Coreceptors are obvious candidates, but because they are usually required for MHC recognition it has been difficult to independently assess their role in lineage choice. Our findings demonstrate that at least some thymocytes with MHC class II specific TCR are CD4 independent in their recognition of class II and enabled us to separately examine the role of the coreceptor in lineage commitment. T cells with these TCRs develop in the CD4 pathway when CD4 is present but in the CD8 pathway when CD4 is absent. Thus, even for a TCR that is coreceptor independent, the coreceptor can influence the lineage choice. From these results we have proposed a new instructional model in which lineage choice is determined by the intensity of signals delivered to CD4 and CD8 thymocytes through the TCR complex. There are no distinct signals but rather it is the net quantity of TCR signals that direct the lineage choice. We are currently testing whether signals of strong intensity promote the CD4 fate, whereas weaker signals promote a CD8 fate.